Gaps are empty spaces between the close of one candle and the open of another.
Gaps are not very common and usually only occur at the market open on Sundays (with respect to the forex markets). These gaps occur between a pair's close price, on Friday and its open price on Sunday.
"Up Gaps" often fill due to profit-taking from buyers who have open positions, before the Gap and sell, as price moves up.
"Down gaps" often fill due to profit-taking from short-sellers who have open positions, before the gap and exit their short trade, by buying as price moves down.